Methods and apparatuses having a voltage generator with an adjustable voltage drop for representing a voltage drop of a memory cell and/or a current mirror circuit and replica circuit

ABSTRACT

Apparatus and methods utilize a replica circuit to generate a voltage for programming of a memory cell, such as a memory cell of a phase-change memory (PCM). Current passing through a circuit including the memory cell to be programmed is mirrored in a scaled or unscaled manner, and provided as an input to the replica circuit. The replica circuit represents voltage drops that should be encountered when programming the memory cell. An input voltage is also provided to the replica circuit, which affects the voltage drop within the replica circuit that represents the voltage drop of the cell. The voltage drop across the replica circuit can then be mirrored and provided to bias the circuit including the memory cell.

CROSS-REFERENCE TO RELATED APPLICATION

The present Application for Patent is a divisional of U.S. patentapplication Ser. No. 15/054,984 by Di Vincenzo et al., entitled “Methodsand Apparatuses Having a Voltage Generator with an Adjustable VoltageDrop for Representing a Voltage Drop of a Memory Cell and/or A CurrentMirror Circuit and Replica Circuit,” filed Feb. 26, 2016, now U.S. Pat.No. 9,728,256, issued on Aug. 8, 2017, which is a continuation of U.S.patent application Ser. No. 13/800,622 by Di Vincenzo et al., entitled“Methods and Apparatuses Having a Voltage Generator with an AdjustableVoltage Drop for Representing a Voltage Drop of a Memory Cell and/or aCurrent Mirror Circuit and Replica Circuit,” filed Mar. 13, 2013, nowU.S. Pat. No. 9,281,061, issued on Mar. 8, 2016, which claims priorityto U.S. Provisional Patent Application No. 61/703,168 by Di Vincenzo etal., entitled “Voltage-Mode Programming Technique for Phase-ChangeMemory,” filed Sep. 19, 2012, assigned to the assignee hereof, and eachof which is expressly incorporated by reference in its entirety herein.

BACKGROUND Field of the Invention

Embodiments of the invention generally relate to electronics, and inparticular, to apparatuses that include memory cells such asphase-change memory (PCM) cells.

Description of the Related Art

Phase-change memory (PCM), also known as PCRAM or PRAM, is a form ofnonvolatile memory made from a phase-change material. An example of aphase-change material is Germanium-Antimony-Tellurium (GST). When a cellof GST material is heated beyond its melting point and cools relativelyrapidly, the phase-change material of the cell is in an amorphous stateand can have a resistance associated with a first state, such as logic0. When the cell of GST material is heated and is cooled relativelyslowly, the phase-change material of the cell is in a crystalline stateand can have a resistance associated with a second state, such aslogic 1. The cell of GST material can also be heated to a relativelylower temperature, such as a temperature associated with crystallizationto achieve the crystalline state. Intermediate states are also possiblefor multi-level cells.

For relatively good density and low cost, the individual cells of amemory device will be small. However, perfect uniformity duringproduction is not practical, and even relatively small differences insize can result in a relatively large amount of variability in theappropriate amount of power that should be used to heat a cell. Inaddition, other sources of variability exist. Some sources ofvariability can affect an entire array of cells. While it is possible tocompensate for some sources of variability by a calibration procedureperformed during manufacturing, such procedures tend to be timeconsuming, which can increase the cost of production.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings and the associated description herein are provided toillustrate specific embodiments of the invention and are not intended tobe limiting.

FIG. 1 illustrates voltage drops that can be encountered when setting astate for a phase change memory (PCM) cell.

FIG. 2 illustrates an example of a matrix of arrays of PCM cells.

FIG. 3 illustrates a portion of an array.

FIG. 4 illustrates a simplified model of accessing two cells in anarray.

FIG. 5 illustrates a top-level view of an embodiment of the invention.

FIG. 6 illustrates a top-level view of a programming voltage generatorcapable of programming two cells of the same word line at a time.

FIG. 7 illustrates one embodiment of a PCM replica circuit.

FIG. 8 illustrates a voltage drop V_(RMP) across a cell.

FIG. 9 illustrates one embodiment of a ramp adder circuit.

FIG. 10 illustrates one embodiment of a current mirroring and voltagebiasing circuit.

FIG. 11 illustrates an embodiment of a current mirroring and voltagebiasing circuit that combines current minoring and voltage biasing.

FIG. 12 illustrates an example of a circuit that can be used to assistin starting of a feedback loop.

FIG. 13 illustrates a top-level view of a programming voltage generatorwith fast ramping capability.

FIG. 14 illustrates a modulator according to an embodiment of theinvention.

FIG. 15 illustrates an example of a ramping waveform.

FIG. 16 illustrates an example of current versus time due to an inputsignal having a ramping waveform.

FIG. 17 illustrates simulation results of one embodiment of theinvention.

FIG. 18 illustrates simulation results of the conventional art.

To avoid repetition of description, components having the same orsimilar function may be referenced by the same reference number.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Although particular embodiments are described herein, other embodimentsof the invention, including embodiments that do not provide all of thebenefits and features set forth herein, will be apparent to those ofordinary skill in the art.

A phase-change memory cell can have a programmable element having astate that is crystalline, amorphous, or in-between. While described inthe context of a single-level PCM cell with only a crystalline or anamorphous state, the principles and advantages described herein are alsoapplicable to multi-level cells programmable to a target data state of aplurality of potential data states, including cells programmable tostates which are partially crystalline and partially amorphous. Whileillustrated in the context of phase change memory, the principles andadvantages described herein are applicable to other types of memoriesincluding those in existence and those yet to be developed.

There are two fundamental modes for electrically setting a state of aPCM cell. A typical PCM memory device will utilize only one of thesemodes. The first technique is known as voltage mode and the secondtechnique is known as current mode. In the voltage mode, a relativelyconstant voltage is applied to a cell via, for example, column and rowdecoders, such that the cell melts, and then the voltage is ramped down.When the voltage is ramped down relatively slowly (small slope), thestate of the PCM cell is crystalline (typically logic 1). When thevoltage is ramped down relatively quickly (large slope), the state ofthe PCM cell is amorphous (typically logic 0). In one non-limitingexample, a slope smaller than 1 volt (V) per 20 nanoseconds (ns) canresult in a crystalline state (typically logic 1) and a slope largerthan 1V per 20 ns can result in an amorphous state.

Similarly, in the current mode, a relatively constant current is appliedto the cell such that the cell melts, and then the current is rampeddown. The slope of the ramp down determines the state of the PCM cell.Generally, the voltage mode is more favorable than the current modebecause of mismatches in PCM cell geometry. These mismatches can causevariations in current density when the current mode is used. However, inthe voltage mode, these cell geometry mismatches typically do not giverise to variations in current density. Thus, the voltage mode can bemore favorable than the current mode, and PCM devices typically use thevoltage mode.

FIG. 1 illustrates voltage drops that can be encountered whenprogramming (e.g., setting) a state of a PCM cell in the voltage mode.In the illustrated example, the PCM cell is part of an array or tile andis accessed via column and row decoders. The programming path startswith a programming voltage V_(o), has a drop V_(PGL) across a modulator(e.g., PGLOAD system) 104, a drop V_(Ydec) across a column (e.g., bitline) decoder 106, a drop V_(BL) across the resistance of the bit line108, a drop V_(cell) across the PCM cell (e.g., including programmableelement 102 and a selector 110), a drop VwL across the resistance of theword line 112, and a drop Vxdec across a row (e.g., word line) decoder114. The modulator 104 can be used by multiple bit lines and can beconsidered to be outside of the array. While a diode is shown in FIG. 1for the selector 110, other devices can alternatively be used for theselector 110, such as, but not limited to, a metal oxide semiconductorfield effect transistor (MOSFET) or insulated gate field effecttransistor (IGFET) or a bipolar junction transistor (BJT) canalternatively be used. The field-effect transistors (FETs) or“transistors” described herein can correspond to transistors known asmetal-oxide-semiconductor field-effect transistors (MOSFETs). While theterms “metal” and “oxide” are present in the name of the device, it willbe understood that these transistors can have gates made out ofmaterials other than metals, such as polycrystalline silicon, and canhave dielectric “oxide” regions made from dielectrics other than siliconoxide, such as from silicon nitride or high-k dielectrics.

A programming voltage Vo in an illustrative embodiment can be arelatively constant voltage used for melting the PCM cell. The modulator104 can generate a voltage drop in a relatively fast ramped manner, forexample, for ramping down the voltage applied to the PCM cell. As willbe described later in connection with FIG. 14, one embodiment of themodulator 104 has at least a first mode and a second mode of operation,wherein in the first mode, the modulator 104 is configured to provide ashort circuit between the first node and the second node, and wherein inthe second mode, the modulator is configured to generate a voltage dropin a ramped manner between the first node and the second node inresponse to a ramped signal. While the drop V_(cell) is the voltage dropof interest, it is typically not directly observable. It would beimpractical to monitor the drop V_(cell) directly for each PCM cellbecause of the number PCM cells.

The other voltage drops can be appreciable and can thus affect thevoltage drop V_(cell) across the PCM cell. For example, in oneembodiment, the voltage drops V_(Ydec) and V_(xdoc) can be about half ofthe programming voltage V_(o). In addition, the PCM cell is subject tocell geometry variability during manufacture, which affects the dropV_(cell). Further, the other voltage drops can vary over temperature,process variation, and the particular position of the PCM cell within anarray. For example, the position of a particular cell can greatly affectthe amount of resistance of the word line 112 for that particular cell.

FIG. 2 illustrates an example of a matrix of arrays of PCM cells. Thearrays can be arranged in a tile-like manner and can be referred to astiles. The terms array and tile are synonymous, and the term “array”will be used from this point forward. Typically, relatively many PCMcells are arranged in an array 202, and relatively many arrays arearranged into a matrix for reading or writing in parallel and/orincreasing the number of memory cells in a memory device. While 16arrays are shown in the illustrated matrix, the number of arrays canvary in a broad range, and the matrix itself can be considered an array.There can be variation among the cells of the arrays.

FIG. 3 illustrates a portion of the array 202. The array 202 hasrelatively many bit lines (BL) and word lines (WL). For example, thenumber of bit lines and word lines can be in the many thousands perarray. To access a particular cell, at least the bit line and word linecorresponding to that cell are activated (e.g., depending on theassociated logic, a line could be either active high or active low). Twoor more PCM cells of a word line can also be programmed in parallel. Theactivation of more than one PCM cell per word line further increases theuncertainty in the voltage drops associated with the word lines, such asresistive voltage drops. In addition, the resistance of the cell itselfcan change during a programming operation, which can then disturb theprogramming of other cells tied to the same word line. Thus, it can bedifficult to determine a relatively good programming voltage V_(o) touse. A manufacturer can spend a great deal of time selecting a level forthe programming voltage V_(o) during the testing of a chip. However, dueto the variability discussed above, the programming voltage Vo willlikely be good for only some of the cells, which can reduce yield,thereby increasing cost. In addition, time consuming operations, such asthose used during manufacturing test to find a relatively goodprogramming voltage V_(o), can increase the production cost.

FIG. 4 illustrates a simplified model 400 of accessing two cells at atime in an array. It will be understood that more than two cells can beaccessed at the same time. Nonetheless, FIG. 4 illustrates issues thatcan arise when accessing more than one cell at a time per word line.Each of a first cell 402 and a second cell 404 are on separate bit linesBL_A, BL_B, respectively. It should be noted that while there can bethousands of bit lines per word line, the first cell 402 and the secondcell 404 share the same word line in this example.

The model 400 uses resistances to model many of the elements of anarray. Circuits, such as column decoders, row decoders, and/ormodulators will typically be implemented with MOS transistors. Whenthese MOS transistors are “on,” they are associated with an “on”resistance typically referred to as a drain-to-source resistanceR_(Dson). In the illustrated example, the word line crosses the lengthof the array and is decoded at both ends as indicated by the modeledresistances Rdec appearing at the left end and the right end. The wordline can be relatively long. For simplicity, the resistance of the wordline is modeled as three resistances: Rw11, Rw12, and Rw13. Aspreviously noted, a word line can be relatively long and can beconnected to thousands of cells. The resistance of the word line that isencountered by a particular cell depends on the cell's position alongthe word line. This can further increase the difficulty in accuratelysetting the voltage V_(cell) (FIG. 1) across a cell for programming. Inone embodiment, the programming voltages V_(HY) _(_) _(A), V_(Hy) _(_)_(B) are buffered to generate the programming voltages VA and V_(B)having the same magnitudes as the programming voltages V_(HY) _(_) _(A),V_(HY) _(_) _(B).

FIG. 5 illustrates a top-level view of a programming voltage generator502 according to an embodiment of the invention. The illustratedprogramming voltage generator 502 includes a current mirroring circuit504, a voltage biasing circuit 506, and a PCM replica circuit 508. Eachof these elements can be integrated into an integrated circuit for amemory device. Embodiments of the current mirroring circuit 504 and thevoltage biasing circuit 506 will be described in greater detail later inconnection with FIGS. 10 and 11. It should be noted that the currentmirroring circuit 504 and the voltage biasing circuit 506 can becombined as illustrated in FIG. 11. Further details of a two bit lineversion of the PCM replica circuit 508 will be described later inconnection with FIG. 7.

The programming voltage generator 502 operates via a feedback loop. In afirst part of the feedback loop, current I_(ARRAY) passing through a PCMarray 510 is mirrored as current I_(MIRROR) and passed through the PCMreplica circuit 508. The PCM replica circuit 508 uses the currentI_(MIRROR) and an input voltage V_(R) to generate the replicaprogramming voltage V_(REPLICA). In a second part of the feedback loop,the replica programming voltage V_(REPLICA) is buffered by the voltagebiasing circuit 506 to generate a programming voltage V_(ARRAY), whichin turn helps to generate the current I_(ARRAY). The voltage biasingcircuit 506 also maintains the same current I_(ARRAY) as an input and asan output such that the current mirroring circuit 504 can observe thesame current flowing through the PCM array 510. Such feedback loops canbe difficult to start, and starting the feedback loop will be describedlater in connection with FIG. 12. FIG. 5 will now be described ingreater detail.

The programming voltage generator 502 generates a programming voltageV_(ARRAY) for the PCM array 510 from the voltage V_(H), which can befrom a power supply voltage rail. Depending on the configuration, theprogramming voltage V_(ARRAY) can be similar to the programming voltageV_(o) (FIG. 1) (see Eq. 1) or can be slightly different by incorporatingthe function (including the voltage drop V_(PGL)) of the modulator(FIG. 1) (see Eq. 2), an example of which will be described later inconnection with FIG. 14.V _(ARRAY) =V _(PGL) +V _(Ydec) +V _(BL) +V _(cell) +V _(WL) +V_(Xdec)  Eq. 1V _(ARRAY) =V _(Ydec) +V _(BL) +V _(cell) +V _(WL) +V _(Xdec)  Eq. 2

When the programming voltage V_(ARRAY) is applied to a programmingvoltage node V_(HY), a current I_(ARRAY) flows through the node. Thecurrent mirroring circuit mirrors the current I_(ARRAY) to generate amirrored current I_(MIRROR). The mirrored current I_(MIRROR) is providedas an input to the PCM replica circuit 508. In the illustratedembodiments, the mirroring is performed without scaling. In alternativeembodiments, the currents can be scaled when mirrored with appropriatescale changes to the resistances.

The PCM replica circuit 508 comprises circuits that represent (e.g.,replicate or emulate, whether directly or via scaling) the voltage dropsof the programming path described earlier in connection with FIG. 1 andincludes a controlled drop across the PCM cell. The voltage drops can begenerated by passing the mirrored current I_(MIRROR) (which can be thesame as I_(ARRAY) or a scaled version of the same) through the same orscaled decoders, resistances for bit lines and word lines, and the like.The voltage drop V_(cell) across the replica of the PCM cell can be setby a voltage source and controlled by an input voltage V_(R) as will bedescribed later in connection with FIG. 9. The resulting replicaprogramming voltage V_(REPLICA) s then at an appropriate voltage levelto apply as the programming voltage V_(ARRAY) to have the voltage VRapplied across the PCM cell. This can be performed without expensiveprocedures at manufacturing test. While the PCM replica circuit 508 isdrawn in FIG. 5 with separate inputs for the current I_(MIRROR) and theinput voltage V_(R) and a separate output for the replica programmingvoltage V_(REPLICA), it should be noted that the input for the currentI_(MIRROR) and the output for the replica programming voltageV_(REPLICA) can be at the same node. It should be noted that the PCMreplica circuit 508 can be shared among many different cells and indeed,even among different arrays. However, for programming in parallel, morethan one PCM replica circuit 508 will typically be present.

It can be advantageous to program multiple cells in parallel. The tilingof arrays illustrated in FIG. 2 is one way to program in parallel.Another way of programming in parallel is within an array, such as byprogramming two different cells of the same word line at the same time.These two cells will have different bit lines.

FIG. 6 illustrates a top-level view of a programming voltage generator602 capable of programming two cells of the same word line of a PCMarray 604 at a time. Each of these elements can be integrated into anintegrated circuit for a memory device. The basic operation is similarto that described earlier in connection with FIG. 5; however, two PCMcells can share the same word line and there can be more than one rowdecoder per word line. While illustrated in the context of programmingtwo cells, the principles and advantages described herein are alsoapplicable to the programming of more than two cells at the same timeper word line. There can be variability in the voltage drops across thedecoders and in addition, based on a cell's position along the wordline, there can be variability in the amount of word line resistancethat is applicable for a particular cell. Thus, two differentprogramming voltages V_(HY) _(_) _(A), V_(HY) _(_) _(B) are generated,and two different currents I_(A), I_(B) are mirrored as explainedfurther below.

The PCM array 604 can correspond to the model 400 described earlier inconnection with FIG. 4. As modeled in FIG. 4, programming voltagesV_(HY) _(_) _(A), V_(HY) _(_) _(B) give rise to currents I_(A), I_(B).These currents I_(A), I_(B) each flow through a particular columndecoder, through the modulator, bit line resistance, and respective cell402, 404. The currents I_(A), I_(B) then flow through a common word lineand can flow through more than one row decoder. The currents I_(A),I_(B) are mirrored by current minoring circuits 606, 616 to generatemirrored currents I_(AM), I_(BM), which are provided as inputs to a PCMreplica circuit 610. The current mirroring circuits 606, 616 can be thesame as the current mirroring circuit 504 described earlier inconnection with FIG. 5.

The mirrored currents I_(AM), I_(BM) generate voltage drops acrossresistances replicating resistances of the PCM array 604. In addition,an input voltage V_(R) is provided as an input to the PCM replicacircuit 610 and contributes to the replica programming voltages V_(HY)_(_) _(A), V_(HY) _(_) _(B) generated as an output of the PCM replicacircuit 610. One embodiment of the PCM replica circuit 610 will bedescribed in greater detail later in connection with FIG. 7. The voltagebiasing circuits 608, 618 are in series with the mirror currents I_(AM),I_(BM) and buffer the replica programming voltages V_(HY) _(_) _(A),V_(HY) _(_) _(B) to generate the programming voltages V_(A), V_(B),which are provided as inputs to the PCM array 604. The voltage biasingcircuits 608, 618 can be the same as the voltage biasing circuit 506described earlier in connection with FIG. 5. In addition, it should benoted that a current mirroring circuit can be combined with a voltagebiasing circuit as illustrated in FIG. 11, or can be separate circuitsas illustrated in FIG. 10. Further, it should be noted that one node canbe used as an input for the current I_(AM) and an output for the replicaprogramming voltage V_(HY) _(_) _(A), and another node can be used as aninput for the current IBM and an output for the replica programmingvoltage V_(HY) _(_) _(B). For programming in parallel, more than one PCMreplica circuit 610 will typically be present in a memory device.However, the PCM replica circuit 610 can be shared among many differentbit lines, word lines, and even different arrays.

FIG. 7 illustrates further details of one embodiment for the PCM replicacircuit 610. Two different programming voltages VHy A, VHy H for twodifferent cells A, B of the same word line are generated. The PCMreplica circuit 610 includes replica circuits 702, 712 for themodulator, replica circuits 704, 714 for column decoders, ramp addercircuits 706, 716, switches 708, 718, replica resistances 709 for theword line, and replica circuits 710, 720 for the row decoders. In oneembodiment, these replica circuits or resistances are the same as thatexpected for the corresponding components of the PCM array 604. However,the replica resistances and the currents I_(AM), I_(BM) canalternatively be scaled. The replica circuits can be implemented byresistors or by active devices, such as devices similar to those used inthe output stages of a column decoder or row decoder. For example, whenno scaling is used, the replica circuits 704, 714, 710, 720 of thecolumn decoder and the row decoder can be the same as the outputcircuits of the column decoders and the row decoders of the array. Inone embodiment, the replica circuits 702, 712 replicate only theon-resistance drop of the modulator when the modulator is behaving as ashort circuit, which will be explained later in connection with FIGS.14-16. While drawn as part of the PCM replica circuit 610, the replicacircuits 702, 712 of the modulator can also be considered to be outsideof the PCM replica circuit 610.

In the illustrated embodiment, 6 resistances 709 replicate theresistance of a word line of the PCM array 604. There can be many, suchas, but not limited to, thousands of cells per word line. Accordingly,the resistance of the word line can vary depending on a cell's position.A transistor from among the transistors 708, 718 is activated to selectan appropriate node in the replica resistance 709 to represent theactual resistance encountered by a cell based on the cell's positionalong its word line. For example, when programming a cell close to a rowdecoder corresponding to the row decoder replica resistance 710, atransistor from among the transistors 708 switched on that is close tothe replica resistance 710 can be switched on. By contrast, whenprogramming a cell close to the middle of the word line, a transistorfrom among the transistors 708 that is close to the middle of theresistances can be activated. For cells in between an end and themiddle, a transistor intermediate between the two can be switched oninstead. While illustrated with 6 resistances, the number of resistancescan vary in a very broad range. For example, 2, 4, 8, 10 or othernumbers of resistances can be used. The resistance of the transistors708 can be made negligible by sizing the transistors 708 relativelylarge or can be taken into account with the other replica resistances.Since there are relatively few replica circuits needed as compared tocells, the size of the components of a replica circuit is not ascritical as it is in an array, and the transistors 708 can be maderelatively large without much of a cost penalty.

To generate the programming voltages VHy A, VHY B, currents I_(AM)I_(BM) are applied from current sources 722, 724 to the node generatingthe programming voltages VHy A, VHy B, and an input voltage V_(R) isapplied to the ramp adder circuits 706, 716. In one embodiment, thecurrent sources 722, 724 are provided by the current mirroring circuits606, 616 (FIG. 6), example embodiments of which will be described ingreater detail later in connection with FIGS. 10 and 11. The currentsI_(AM), I_(BM) generate voltage drops across the applicable replicaresistances. For example, the current I_(AM) generates a voltage dropacross the replica resistances 702, 704, 710 and some part of the wordline replica resistance 709. It should be noted that since both thecurrent I_(AM) and the current I_(BM) flow through the same word linereplica resistance 709, there is interaction between the two programmingvoltages VHy A, VHy B similar to the interaction found on the PCM array604.

In the illustrated embodiment, the ramp adder circuits 706, 716 generatean adjustable voltage drop V_(RMP) equal in magnitude to the inputvoltage V_(R). However, there can be gain or loss, for example, scaling,in alternative embodiments such that the magnitude of the adjustablevoltage drop V_(RMP) and the input voltage V_(R) do not necessarily haveto be equal. The adjustable voltage drop V_(RMP) is an adjustablevoltage drop for representing the voltage drop of the cell (e.g., acrossa programmable element and a selector), and should be equal to thedesired level for the voltage V_(cell) to be dropped across a cell asshown in FIG. 8 and earlier in FIG. 1. In one embodiment, the inputvoltage V_(R) is referenced to a voltage reference, such as ground,while the adjustable voltage drop V_(RMP) is a voltage drop that isfloating due to the voltage drops across the replica resistances for theword line and the row decoder. Of course, the adjustable voltage dropV_(RMP) can vary based on the cell geometry, cell material, and the typeof selector used. In the illustrated embodiment, the adjustable voltagedrop V_(RMP) is about 3.8 volts, however, other voltages can be used.For example, in other embodiments, the adjustable voltage drop V_(RMP)can be in the range of 3.7V to 3.9V, or 3.6V to 4.0V, or 3.5V to 4.2V.Other applicable voltages will be readily determined by one of ordinaryskill in the art.

FIG. 9 illustrates one embodiment of a ramp adder circuit 900. The rampadder circuit 900 can be used for the ramp adder circuit 706 or the rampadder circuit 716 of FIG. 7. The ramp adder circuit 900 generates anadjustable voltage drop V_(RMP) that is floating with respect to a firstnode BL_1 and a second node BL 2, and maintains the current I_(AM)passing through the ramp adder circuit 900. The adjustable voltage dropV_(RMP) can be about equal to an input voltage V_(R) that is referencedto a voltage reference, such as ground or VSS. In alternativeembodiments, there can be scaling, such as gain or loss, between themagnitudes of the adjustable voltage drop V_(RMP) and the input voltageV_(R).

The ramp adder circuit 900 includes a first amplifier 902, a firstN-type MOS (NMOS) transistor 904, a first resistor 906, a first P-typeMOS (PMOS) transistor 908, a second PMOS transistor 910, a secondresistor 912, a second amplifier 914, and a third PMOS transistor 916.The operation of the ramp adder circuit 900 will now be described. Inthe following description, it is assumed that input currents of theamplifiers and gate currents of the transistors are negligible.

The voltage V_(BL) _(_) ₂ from the node BL_2 is provided as an input toa non-inverting input of the first amplifier 902, which can be anoperational amplifier. The output of the first amplifier 902 drives agate of the first NMOS transistor 904. A source of the first NMOStransistor 904 is coupled to an inverting input of the first amplifier902 and to a first end of the first resistor 906. Due to feedback, thenode of the source of the first NMOS transistor 904 will be at a voltagethat is about the same as the voltage V_(BL) _(_) ₂. Ignoring currentthrough the non-inverting input of the first amplifier 902 as beingnegligible, a current I passes through the first resistor 906 having avalue of the voltage V_(BL) _(_) ₂ divided by the resistance R. The samecurrent I passes through the first NMOS transistor 904 and through thefirst PMOS transistor 908.

The first PMOS transistor 908 is connected in a diode configuration withits gate tied to its drain. The sources of the first PMOS transistor 908and the second PMOS transistor 910 are tied to a power supply voltagebias VH. The gates of the first PMOS transistor 908 and the second PMOStransistor 910 are tied to each other. Thus, the first PMOS transistor908 and the second PMOS transistor 910 have the same gate-to-sourcevoltages such that the first PMOS transistor 908 and the second PMOStransistor 910 form a current mirror. In the illustrated embodiment, thefirst PMOS transistor 908 and the second PMOS transistor 910 are alsomatched in geometry. Accordingly, the same amount of current I flowsthrough the second PMOS transistor 910 and the first PMOS transistor908.

A first end of the second resistor 912 is coupled to the drain of thesecond PMOS transistor 910, and a second end of the second resistor iscoupled to a node providing the input voltage V_(R). The current I flowsthrough the second resistor 912, which has the same resistance R and thesame current I as the first resistor 906. Thus, the voltage drop of Itimes R across the second resistor 912 is the same as the voltage dropacross the first resistor 906, that is, about voltage V_(BL) _(_) ₂. Thedrop of V_(BL) _(_) ₂ when added to the input voltage V_(R) existing atthe second end of the second resistor 912 yields a voltage ofV_(R)+V_(BL) _(_) ₂ at the node of the first end of the second resistor912, drain of the second PMOS transistor 910, and non-inverting input ofthe second amplifier 914.

The second amplifier 914 can be an operational amplifier. An output ofthe second amplifier 914 is coupled to a gate of the third PMOStransistor 916. A source of the second amplifier 914 is coupled to thenode BL 1, and a drain of the second amplifier 914 is coupled to thenode BL_2. The inverting input of the second amplifier 914 is alsocoupled to the node BL 1. Due to feedback, the second amplifier 914provides a gate voltage to the second amplifier 914 such that thevoltage at the node BL_1 is about equal to V_(R)+V_(BL) _(_) ₂. Recallthat the node BL_2 has the voltage V_(BL) _(_) ₂. Thus, it can beobserved that the voltage drop from source to drain of the third PMOStransistor 916 is a floating adjustable voltage V_(RMP) having the samemagnitude as the input voltage V_(R).

Since the cell voltage V_(cell) can now be relatively preciselycontrolled, in one embodiment, the input voltage V_(R) can further taketemperature into account. For example, a typical PCM device will have atemperature sensor embedded therein. The voltage V_(cell) applied to acell to melt the cell should vary with temperature for betterperformance. The input voltage V_(R) can be compensated overtemperature. For example, a lookup table can be used to storeappropriate settings for the input voltage V_(R), which can then haveits level set by a controller of a digital-to-analog converter. Thecontroller can be implemented by a microprocessor executing programinstructions, a microcontroller executing program instructions, a logiccircuit, a combination of the foregoing, or the like.

Alternatives and variations of the circuit of FIG. 9 exist. For example,in the illustrated embodiment, the same amount of resistance is used forthe first resistor 906 and the second resistor 912 and mirroring withoutscaling is used for the currents through the first PMOS transistor 908and the second PMOS transistor 910. In an alternative embodiment, theresistances and currents can be scaled. For example, in an alternativeembodiment, the first resistor 906 can have half the resistance of thesecond resistor 912, and the current passing through the second PMOStransistor 910 can then have half the current passing through the firstPMOS transistor 908 to yield the same voltage drops. For example, if thesecond PMOS transistor 910 has half the effective width of an otherwiseidentical first PMOS transistor 908, then the current mirror will bescaled by half. In another example, a complementary version of at leasta portion of the circuit can be applicable. For example, the third PMOStransistor 916 can be replaced by an NMOS transistor and thenon-inverting and inverting inputs of the amplifier 914 swapped.

FIG. 10 illustrates one embodiment of a current mirroring and voltagebiasing circuit 1000. A first PMOS transistor 1002 and an amplifier 1004form a voltage biasing circuit that can be used for the voltage biasingcircuits 506, 608, 618. The labeling in FIG. 10 corresponds to thevoltage biasing circuit 608 of FIG. 6. A second PMOS transistor 1006 anda third PMOS transistor 1008 form a current mirroring circuit that canbe used for the current mirroring circuit 504, 606, 616. The operationof the current mirroring and voltage biasing circuit 1000 will now bedescribed. In the following description, it is assumed that inputcurrents of the amplifiers and gate currents of the transistors arenegligible.

Sources of the second PMOS transistor 1006 and the third PMOS transistor1008 are coupled to the power supply bias voltage V_(H). A gate and adrain of the second PMOS transistor 1006 are coupled together in a diodeconfiguration. A gate of the third PMOS transistor 1008 is coupled tothe gate of the second PMOS transistor 1006 such that the second PMOStransistor 1006 and the third PMOS transistor 1008 have the samegate-to-source voltage. With the second PMOS transistor 1006 and thethird PMOS transistor 1008 matched and having the same gate-to-sourcevoltage, the same amount of current IA flows through each transistor1006, 1008, with the current flowing through the second PMOS transistor1006 controlling the amount of current IA.

The drain of the second PMOS transistor 1006 is also coupled to thesource of the first PMOS transistor 1002. The drain of the first PMOStransistor 1002 is coupled to a non-inverting input of the amplifier1004. The amplifier 1004 can be an operational amplifier. Due to thephase inversion from the gate to the drain of the first PMOS transistor1002, the feedback loop uses the non-inverting input of the amplifier1004 as the input for negative feedback. The output of the amplifier1004 is coupled to the gate of the first PMOS transistor 1002. A voltageV_(HY) _(_) _(A) from the PCM replica circuit 610 is provided as aninput to the inverting input of the amplifier 1004. It should be notedthat the same node can be used to receive the current I_(AM) and toprovide the voltage V_(HY) _(_) _(A). The operation of the feedback loopprovides that the amplifier 1004 adjusts the gate voltage of the firstPMOS transistor 1002 such that the programming voltage V_(A) on thedrain of the first PMOS transistor 1002 is about the same as the voltageV_(HY) _(_) _(A). The programming voltage V_(A) is provided as an inputto the PCM array 604.

FIG. 11 illustrates an embodiment of a current mirroring and voltagebiasing circuit 1100 that combines current mirroring and voltagebiasing. One advantage that the current minoring and voltage biasingcircuit 1100 has over the current mirroring and voltage biasing circuit1000 is that the current minoring and voltage biasing circuit 1100 canoperate with a smaller voltage drop since there is only one transistorin the current path versus two transistors. The labeling in FIG. 11corresponds to the voltage biasing circuit 608 of FIG. 6.

The current mirroring and voltage biasing circuit 1100 includes a firstPMOS transistor 1102, a second PMOS transistor 1104, and an amplifier1106. The sources of the first PMOS transistor 1102 and the second PMOStransistor 1104 are coupled to a power supply bias voltage VH. The gatesof the first PMOS transistor 1102 and the second PMOS transistor 1104are coupled to a node MIRR, which is driven by the output of theamplifier 1106. With the first PMOS transistor 1102 and the second PMOStransistor 1104 matched and sharing the same gate-to-source voltage, thecurrent IA passing through the first PMOS transistor 1102 and thecurrent I_(AM) passing through the second PMOS transistor 1104 should beabout the same. The PCM replica circuit 610 generates the voltage V_(HY)_(_) _(A), which is provided as an input to an inverting input of theamplifier 1106.

The phase inversion going from gate to drain for the first PMOStransistor 1102 and the second PMOS transistor 1104 results in thenon-inverting input being the input for negative feedback. The operationof the feedback loop provides that the programming voltage VA that ispresent at the drain of the first PMOS transistor 1102 and at thenon-inverting input of the amplifier 1106 is about equal to the voltageV_(HY) _(_) _(A). The programming voltage VA is provided as an input tothe PCM array 604.

FIG. 12 illustrates an example of a circuit that can be used to assistin starting of a feedback loop. The circuit is illustrated in thecontext of the current mirroring and voltage biasing circuit 1000 (FIG.10) and includes a first NMOS transistor 1202 and, in at least in someembodiments, a second NMOS transistor 1204. The gate of the first NMOStransistor 1202 is tied to the input voltage V_(R), the drain to thevoltage VH, and the source to the node with the voltage V_(HY) _(_)_(A). The first NMOS transistor 1202 operates as a source follower andthus biases the node with the voltage V_(HY) _(_) _(A) to a voltage ofthe input voltage V_(R) minus a gate-to-source voltage drop. Thisensures that the programming voltage VA starts at a nonzero voltage andprovides some current to the PCM replica circuit 508 or the PCM replicacircuit 610. When the voltage V_(Hy) _(_) _(A) rises on its own, thefirst NMOS transistor 1202 becomes cutoff. Similarly, the second NMOStransistor 1204 can also be in a source follower configuration and canprovide an initial amount of current and voltage biasing. Adding thesecond NMOS transistor 1204 can help provide symmetry and assists tofurther increase the speed at which the feedback loop can be started.The same node with the voltage V_(HY) _(_) _(A) of the current mirroringand voltage biasing circuit 1100 can be biased in a like manner by thefirst NMOS transistor 1202 for starting of the programming voltagegenerator 602 with the current mirroring and voltage biasing circuit1100.

FIG. 13 illustrates a top-level view of a programming voltage generator1302 with fast ramping capability. The programming voltage generator1302 is similar to the programming voltage generator 602 of FIG. 6, butwith the addition of a first modulator block 1304 and a second modulatorblock 1306. To avoid repetition of description, components having thesame or similar function are referenced by the same reference number.The principles and advantages are also applicable to the PCM replicacircuit of FIG. 5.

The first modulator block 1304 and the second modulator block 1306receive the programming voltages V_(A), V_(B) as inputs and generateprogramming voltages V_(AP), V_(BP) as outputs. To ramp the programmingvoltages V_(AP), V_(BP) relatively slowly to generate, for example, acrystalline structure, the first modulator block 1304 and the secondmodulator block 1306 can be short circuits such that the programmingvoltages V_(AP), V_(BP) are the same as the programming voltages V_(A),V_(B), and the ramping is provided by manipulation of the adjustablevoltage drop V_(RMP) within the PCM replica circuit 610, which in turnadjusts the levels of the programming voltages V_(A), V_(B).

However, to ramp the programming voltages V_(AP), V_(BP) relativelyquickly to generate, for example, an amorphous structure, the firstmodulator block 1304 and the second modulator block 1306 modify theprogramming voltages V_(A), V_(B) to generate a ramped profile for theprogramming voltages V_(AP), V_(BP). Fast ramping can make it possibleto set the state of relatively many cells of an array, such as a blockof cells, relatively quickly. The first modulator block 1304 and thesecond modulator block 1306 can be implemented by the circuitillustrated in FIG. 14.

FIG. 14 illustrates a modulator 1400 according to an embodiment of theinvention. The modulator 1400 is shown with the labeling for the firstmodulator block 1304, but the same circuit can be also be used for thesecond modulator block 1306. The illustrated modulator 1400 includes afirst NMOS transistor 1402, a first PMOS transistor 1404, a second NMOStransistor 1406, and a second PMOS transistor 1408. A gate of the firstNMOS transistor 1402 is tied to a signal HIREF, a source of the firstNMOS transistor 1402 is tied to a low-voltage power supply referenceVSS, such as a ground, and a drain of the first NMOS transistor 1402 iscoupled to a drain of the first PMOS transistor 1404. A gate of thefirst PMOS transistor 1404 is tied to a signal V_(PGL) _(_) _(IN), and adrain of the first PMOS transistor 1404 is tied to the programmingvoltage V_(A). A gate of the second NMOS transistor 1406 is tied to thesignal V_(PGL) _(_) _(IN), a drain of the second NMOS transistor 1406 istied to the programming voltage V_(A), and a source of the second NMOStransistor 1406 is coupled to a source of the second PMOS transistor1408. A gate of the second PMOS transistor 1408 is tied to a signal EN_Bor EN (active low), and a drain of the second PMOS transistor 1408provides the programming voltage V_(AP). Since the second PMOStransistor 1408 is used for enabling/disabling, it can be substitutedwith an NMOS transistor with the enable signal changing to active high.

The modulator 1400 has two modes of operation: a first mode in which itprovide a short circuit, and a second mode in which it is configured togenerate a voltage drop in a ramped manner (e.g., a fast rampingfunction) in response to a ramped signal (V_(PGL) _(_) _(IN)). Themodulator 1400 can also be disabled by raising the signal EN_B high;otherwise, for the modes of operation, the signal EN_B should be low orat ground to turn on the second PMOS transistor 1408.

In the first mode of operation, the signal V_(PGL) _(_) _(IN) can beswitched to the power supply bias voltage VH or high and the signalHIREF can also be set to the power supply bias voltage VH. In oneembodiment, the signal HIREF can be permanently set to the power supplybias voltage VH. With the signal V_(PGL) _(_) _(IN) high, the first PMOStransistor 1404 is off, and the second NMOS transistor 1406 is on. Sinceboth the second NMOS transistor 1406 and the second PMOS transistor 1408are on, the programming voltage V_(AP) is effectively shorted to theprogramming voltage V_(A). With the first PMOS transistor 1404 off, thecurrent IBIAS should be zero, and the current IA CELL should be aboutequal to the current IA.

In the second mode of operation, the first NMOS transistor 1402 canagain be turned on by setting the signal HIREF to a high level, and thesecond PMOS transistor 1408 will likewise be turned on by setting theEN_B signal low. The signal V_(PGL) _(_) _(IN) is set to a rampingwaveform such as illustrated in FIG. 15. In one embodiment, the signalV_(PGL) _(_) _(IN) corresponds to a sawtooth waveform as shown in FIG.15 that starts at 0 volts, then jumps to 2 volts, and then ramps backdown to 0 volts in about 25 nanoseconds before repeating. Otherwaveforms can be used. For example, a square wave can be used providedthat the slope of the ramp down (falling edge) has the appropriatecharacteristic. In one non-limiting example, a slope smaller than 1 volt(V) per 20 nanoseconds (ns) can result in a crystalline state (typicallylogic 1) and a slope larger than 1V per 20 ns can result in an amorphousstate.

With the signal V_(PGL) _(_) _(IN) applied to the gate of the secondNMOS transistor 1406 and with the second PMOS transistor 1408 switchedon such that there is negligible voltage loss across the second PMOStransistor 1408, the programming voltage V_(AP) can be based on theoutput voltage of a source follower configuration (in the saturationregion of operation) having an input signal of the signal V_(PGL) _(_)_(IN) as expressed in Equation 3 or, when the signal V_(PGL) _(_) _(IN)is sufficiently high, the second NMOS transistor 1406 can be switched on(in the linear or ohmic region of operation) such that the programmingvoltage V_(AP) is about equal to the programming voltage V_(A).V _(AP) =V _(PGLIN) −V _(TH)  Eq. 3

In Equation 3, the threshold voltage V_(TH) is the threshold voltage ofthe second NMOS transistor 1406. The resulting effect on the current IACELL provided to a PCM cell of the PCM array 604 is illustrated in FIG.16. Time is expressed along a horizontal axis. Voltage and currents areexpressed along a vertical axis. Further, the first PMOS transistor 1404begins to turn on as the voltage of the signal V_(PGL) _(_) _(IN) goeslow such that the current IBIAS begins to flow. Thus, the current IA issplit between the current IA CELL and the current IBIAS. In theillustrated example, the current IBIAS is merely diverted to ground.However, by having the current IBIAS flow, this keeps the current IAflowing even when the current IA CELL goes to zero. The current IA ismirrored for biasing of the PCM replica circuit 610, so that by keepingthe current IA flowing, the PCM replica circuit 610 can remain biased.

FIG. 17 illustrates simulation results of one embodiment of theinvention. FIG. 18 illustrates simulation results of the conventionalart. In each, a voltage of 3.8 volts is the goal for the melting voltagefor the cell. However, it will be understood that 3.8 volts was usedjust for an example and that other voltages may be applicable with cellshaving a different geometry or made of different material. To simulatesome variability, cell B has about 1 kilohm more series resistance thancell A. In addition, cell A is simulated to be near a word line driverand cell B is simulated to be near the middle of an array. Table 1illustrates the various temperatures and various programming models thatwere simulated.

TABLE 1 Condition Temperature (C.) Model Typical 27 typical Slow 85 slowFast −40 fast C1 85 fast C2 −40 slow

As illustrated in FIG. 17, the programming voltages for both Cell A andCell B for an embodiment of the invention are relatively close to thegoal of 3.8 volts. For example, the voltage on Cell A ranges from 3.75volts to 3.83 volts, and the voltage on Cell B ranges from 3.75 volts to3.82 volts.

By contrast, the programming voltages illustrated in FIG. 18 for aconventional circuit are quite variable. For example, the voltage onCell A ranges from 3.52 volts to 4.20 volts, and Cell B ranges from 3.62volts to 4.30 volts.

One embodiment includes an apparatus, wherein the apparatus includes: amemory cell of an array, wherein the cell is configured to store data inat least one of a plurality of states; and a voltage generatorconfigured to generate a programming voltage for programming the cell,wherein the voltage generator is configured to represent voltage dropsencountered when programming the cell of the array, wherein therepresented voltage drops include an adjustable voltage drop forrepresenting the voltage drop of the cell.

One embodiment includes an electronically-implemented method ofprogramming a memory cell, wherein the method includes: generating avoltage for programming a memory cell of an array with a voltagegenerator, wherein the cell is configured to store data in at least oneof a plurality of states; and representing voltage drops encounteredwhen programming the cell of the array in a replica circuit, wherein therepresented voltage drops include an adjustable voltage drop forrepresenting the voltage drop of the cell; wherein the generated voltageis based at least partly on the represented voltage drops.

One embodiment includes an apparatus, wherein the apparatus includes: acontroller configured to issue at least one of a read operation, a writeoperation, a status request, or a command; and a memory devicecomprising: a phase-change memory (PCM) cell of a PCM array, wherein thePCM cell is configured to store data in at least one of a plurality ofstates, wherein the plurality of states includes at least a crystallinestate and an amorphous state; and a voltage generator configured togenerate a voltage for programming the PCM cell, wherein the voltagegenerator is configured to represent voltage drops encountered whenprogramming the PCM cell of the PCM array, wherein the representedvoltage drops include an adjustable voltage drop for representing thevoltage drop of the PCM cell.

Semiconductor memory devices are ubiquitous in apparatuses such asdigital electronics devices and various systems (as used herein, an“apparatus” can interchangeably refer to, for example, circuitry, amemory device or combination of memory devices, an electronic device orcombination of electronic devices, or a system or combination ofsystems). These various systems can include a process configured toexecute instructions and/or access data stored on the memory devices.Examples of apparatuses include, but are not limited to, desktopcomputers, workstations, servers, tablets, laptop computers, digitalcameras, video cameras, digital media players, personal digitalassistants, smart phones, mobile phones, navigation devices, nonvolatilestorage products, kiosks, automotive products, and the like.

The foregoing description and claims may refer to elements or featuresas being “connected” or “coupled” together. As used herein, unlessexpressly stated to the contrary, “connected” means that oneelement/feature is directly or indirectly connected to anotherelement/feature, and not necessarily mechanically. Likewise, unlessexpressly stated to the contrary, “coupled” means that oneelement/feature is directly or indirectly coupled to anotherelement/feature, and not necessarily mechanically. Thus, although thedrawings illustrate various examples of arrangements of elements andcomponents, additional intervening elements, devices, features, orcomponents may be present in an actual embodiment.

As used herein, a “node” refers to any internal or external referencepoint, connection point, junction, signal line, conductive element, orthe like at which a given signal, logic level, voltage, data pattern,current, or quantity is present.

Various embodiments have been described above. Although described withreference to these specific embodiments, the descriptions are intendedto be illustrative and are not intended to be limiting. Variousmodifications and applications may occur to those skilled in the art.

We claim:
 1. A method of programming a memory cell, comprising:determining a voltage drop of a programming path to one or more memorycells of an array, the one or more memory cells each storing one or morelogic states; determining a voltage for a voltage source based at leastin part on the determined voltage drop of the programming path; applyinga programming voltage to the one or more memory cells of the array basedat least in part on the determined voltage, wherein the one or morelogic states are based at least in part on a modulation of thedetermined voltage; mirroring a current received by the voltage sourceusing a current mirroring circuit; transmitting a replica voltage basedat least in part on voltage drops of the programming path to the one ormore memory cells of the array and the mirrored current; and setting theprogramming voltage to a nonzero voltage at a first time using a sourcefollower circuit connected to the current mirroring circuit and avoltage biasing circuit.
 2. The method of claim 1, wherein themodulation of the determined voltage is based at least in part on:adjusting the determined voltage at a modulator; and applying theadjusted voltage to the one or more memory cells.
 3. The method of claim2, further comprising: ramping an input voltage signal received by themodulator, wherein the adjusted voltage is based at least in part on theramped input voltage signal.
 4. The method of claim 3, wherein rampingthe input voltage signal comprises: ramping the input voltage signal ata slope less than 1 volt per 20 nanoseconds to produce a crystallinestate for at least one of the one or more memory cells, or ramping theinput voltage signal at a slope greater than 1 volt per 20 nanosecondsto produce an amorphous state for at least one of the one or more memorycells.
 5. The method of claim 2, further comprising: setting theprogramming voltage to the determined voltage by short-circuiting themodulator.
 6. The method of claim 1, wherein determining the voltage forthe voltage source comprises: applying a second voltage to a replicacircuit that comprises a resistance that is based at least in part onthe resistance of the programming path to the one or more memory cells.